Shekalim, Chapter Eight, Mishnah Three
This mishnah deals with a knife found in Jerusalem around the time of Pesah when many people will need to slaughter their pesach sacrifices. The question is: can he rely on the fact that the knife is pure or must he immerse it before he slaughters with it?
1) A [slaughtering] knife which was found on the fourteenth [of Nisan] he may slaughter with it immediately.
a) [If it was found] on the thirteenth [of Nisan] he must immerse it again.
2) But a chopping knife whether [found] on the fourteenth or on the thirteenth, he must immerse it again.
a) If the fourteenth fell on Shabbat, he may slaughter with it immediately.
b) [If found] on the fifteenth, he may slaughter with it immediately.
3) If [the chopping knife] was found tied to a [slaughtering] knife it may be treated as the knife.
Section one: If the slaughtering knife is found on the fourteenth of Nisan, he may use it to slaughter his pesah immediately, without bringing it to be immersed. In this case we assume that its owners immersed it on the thirteenth, knowing that it would need to be pure the following day, the day on which the pesah is slaughtered.
However, if it was found on the thirteenth we must assume that it has not yet been immersed and therefore is unclean. Even though we established in the previous mishnah that all vessels (which includes knives) are assumed to be pure, when it comes to a vessel which will be used with a sacrifice we must be more stringent.
Section two: The knife described here is not used for slaughtering but for cutting meat and bones of an already slaughtered animal. The mishnah uses the word slaughter with regard to this knife as well, but the intention is not slaughter but to chop. This type of knife will be used during the festival, already on the fifteenth, but it will not be used on the fourteenth.
Since we cannot assume that this type of knife was immersed on the thirteenth of Nisan, it must be immersed on the fourteenth in order to use it on the fifteenth and during the remainder festival.
If it was found on the fourteenth of Nisan which fell on Shabbat, then we can assume that it was immersed on the thirteenth because it is forbidden to immerse vessels on Shabbat. They would have immersed it on the thirteenth so that they could use it to chop meat and bones on the fifteenth and during the remainder of the festival.
If he found it on the fifteenth, he can assume that it was immersed on the fourteenth. Therefore he may use it without immersing it again.
Section three: Finally, if he finds the chopping knife tied to the slaughtering knife, he should treat the chopping knife the same way he treats the slaughtering knife.